[B + C] Adds a villager trading API. Adds BUKKIT-2138, BUKKIT-3935

The Issue:

Currently in bukkit there is no way to get or set merchant trades or get a merchant from its inventory

PR Breakdown:

This PR adds a way to get, set and add merchant trades and adds a way to get the merchant from the MerchantInventory

Testing Results and Materials:

https://github.com/thefishlive/Bukkit-test - source
https://www.dropbox.com/sh/9v7cy8c2acsuywn/mqdvl8kL3S/Bukkit-Test-0.0.1-SNAPSHOT.jar - binary

Relevant PR(s):

CB-#### - https://github.com/Bukkit/CraftBukkit/pull/#### - Implementation (will link when created)

JIRA Ticket: 

https://bukkit.atlassian.net/browse/BUKKIT-2138
https://bukkit.atlassian.net/browse/BUKKIT-3935